1. Technical Field of the Invention
The present invention relates generally to a discharge lamp lighting driver using a piezoelectric transformer which his designed to provide stable emission of light.
2. Background Art
Japanese Patent No. 2767730 discloses a conventional discharge lamp lighting driver. This driver is designed to apply the voltage having a frequency higher than a driving frequency of a piezoelectric transformer required to develop the voltage for igniting a discharge lamp and then sweep that frequency to a lower side until the discharge lamp is ignited. After the discharge lamp is ignited, the lighting driver continues to sweep the frequency of the voltage applied to the piezoelectric transformer to the lower side until the current required to keep stable emission of light from the discharge lamp is reached.
However, if a difference between the driving frequency of the voltage applied to the piezoelectric transformer required to produce the voltage for igniting the discharge lamp and that required to supply the current to the discharge lamp for keeping the stability of lighting of the discharge lamp is great, a discharge lamp lighting driver of the above described type encounters a drawback as will be discussed below.
FIG. 2 illustrates the relation between the current and voltage outputted to the discharge lamp from the piezoelectric transformer and the driving frequency of the voltage applied to the piezoelectric transformer.
The ignition of the discharge lamp is, as shown in the drawing, achieved by applying the voltage having a first frequency a to the piezoelectric transformer to output a first voltage A (X point), to the discharge lamp and lowering the first frequency a to a second frequency b required for stabilizing the lighting of the discharge lamp. The current supplied to the discharge lamp is then kept at B (Y point). If, however, a difference between impedance of the discharge lamp before ignited and that when the discharge lamp is placed in a stable condition is great, it will result in an increased interval between the first frequency a and the second frequency b, which may make it difficult to sweep the driving frequency of the voltage applied to the piezoelectric transformer from a to b within a short period of time. Specifically, the voltage having a frequency c (Z point) which is higher than the frequency b required for the stability of lighting of the discharge lamp is undesirably applied to the piezoelectric transformer. Therefore, after the discharge lamp is ignited, the voltage suitable for keeping the stable emission of light from the discharge lamp is not applied to the piezoelectric transformer, resulting in lack of current supplied to the discharge lamp.